EP0189407B1

EP0189407B1 - Stabilised thermoplastic polymer compositions

Info

Publication number: EP0189407B1
Application number: EP84903650A
Authority: EP
Inventors: Gerald Scott; Khirud Behari Chakraborty
Original assignee: Individual
Current assignee: CHAKRABORTY Khirud Behari; SCOTT Gerald
Priority date: 1983-09-30
Filing date: 1984-10-01
Publication date: 1992-04-29
Anticipated expiration: 2004-10-01
Also published as: GB8326238D0; US4672088A; WO1985001508A1; GB8424762D0; EP0189407A1; JPS61500123A; DE3485690D1; GB2147300A

Abstract

Thermoplastic polymers are stabilised by mixing therewith, under polymer processing conditions such that nitroxyl radicals are formed, a nitrogen-containing stabiliser therefor, which is (a) a C-nitroso-containing compound, (b) an N-nitroso-containing compound, or (c) a nitrone-containing compound.

Description

This invention relates to the stabilisation of thermoplastic polymer compositions.
Most thermoplastic polymers are subject to oxidative deterioration during processing or in service. This deterioration can lead to loss of properties by the polymers, e.g. embrittlement or discolouration, which can be undesirable. Most particularly thermoplastic polymers are subject to deterioration when, in service, they are in an outdoor environment or otherwise subjected to U.V. radiation. In addition deterioration can occur, e.g. as a result of oxidation, when, in service, the polymers are subjected to high temperature. Also the high shearing forces and temperatures used in polymer processing often result in deterioration.
It is well known to incorporate so-called stabilisers into thermoplastic polymer compositions to prevent or retard such deterioration processes. Such stabilisers, according to the manner in which they act, are U.V. stabilisers, thermal antioxidants or melt stabilisers. Amongst the commercially available U.V. stabilisers are the compounds of the formulae

[bis(2,2,6,6-tetramethyl-4-piperidinyl)sebacate]
and

which are sold under the names Tinuvin 770 (ex Ciba-Geigy AG) and Cyasorb UV 531 (ex American Cyanamid) respectively. Such conventional stabilisers, in particular Tinuvin 770, are often expensive to prepare. In order to ensure that a particular polymer composition has the required stability characteristics, it is often necessary to use a mixture of stabilisers. For example there may be added to the polymer both a U.V. stabiliser and a melt stabiliser to protect the polymer from deterioration both during the processing of it and during its service. This of course tends further to add to cost.
The mechanism of the stabilisation is not fully understood and clearly different stabilisers can act in different ways. For example the mechanism by which Tinuvin 770 stabilises thermoplastic polymers is different from that by which Cyasorb UV 531 stabilises them. Recent studies (Bagheri, Chakraborty & Scott, Polymer Degradation and Stability 4, (1982), 1-16) on the use of hindered piperidines, of which Tinuvin 770 is an example, attribute stabilising effectiveness to the oxidation of the hindered piperidines to nitroxyl radicals during the polymer processing and subsequently when the thermoplastic polymer mixture is exposed to U.V. radiation. It is the presence of these nitroxyl radicals which is believed to give rise to the stabilisation.
It has now been found that polyethylene, polypropylene, ethylene-propylene copolymer and polyvinyl chloride may be stabilised against oxidative deterioration by mixing therewith a C-nitroso-containing compound under polymer processing conditions such that there are formed nitroxyl radicals.
According to the present invention, there is provided a process for preparing a stabilised polyethylene, polypropylene, ethylene-propylene copolymer or polyvinyl chloride thermoplastic polymer composition which process comprises mixing the polyethylene, polypropylene, ethylene-propylene copolymer or polyvinyl chloride with a C-nitroso-containing compound as stabiliser under polymer procesing conditions such that nitroxyl radicals are formed.
It is important that the compositions prepared according to the present invention are formed under polymer processing conditions such that nitroxyl radicals are formed. The required conditions are conditions in which the polymer is subjected to high shear in which polymer chains become severed resulting in the mechanochemical formation of hydrocarbyl radicals. Mere mixing without shear, e.g. of polymer and stabiliser solutions, will not provide the U.V. stabilisation properties of the present invention. While we do not wish in any way to be bound by the theory, it is believed that the macrohydrocarbyl radicals and related mechanochemical radicals formed from the polymer in the processing step react with the stabiliser compounds according to the present invention to form the nitroxyl radicals. In particular it has been observed that stabilisation obtained with the compounds according to the present invention is dependant upon the concentration of nitroxyl radicals in the composition after processing. The C-nitroso compounds are believed to react directly with the hydrocarbyl compounds in a radical trapping reaction to form the nitroxyl radicals.
The polymer processing conditions necessary for nitroxyl radical formation according to the present invention depend, in particular, on the thermoplastic polymer used. Generally speaking the polymer and stabiliser are mixed together under high shear at above 150°C, usually 160 to 350°C, and preferably 180 to 280°C in a mixer conventionally used for polymer processing. During the mixing shear is at its highest initially and decreases as the polymer is broken up. It is thus initially that the polyer chains are severed with hydrocarbyl radical formation. It is important according to the present invention that the stabiliser should be present during this high shear to obtain the good results according to the invention. Procesing time also depends on the ingredients being used but will generally be 10 to 20 minutes.
It has been found that the C-nitroso compounds (i.e. compounds containing at least one C - N = O group) have good U.V. stabilising activity for, i.e. they prevent or retard the photo-oxidative embrittlement of, thermoplastic polymers. Also the C-nitroso compounds have melt stabilising activity. The C-nitroso compounds may in particular be aliphatic or aromatic compounds with nitroso-alkanes being preferred. Generally speaking aromatic nitroso compounds are less effective as U.V. light stabilisers but are more effective than their alkyl analogues as thermal antioxidants.
C-nitroso compounds according to the present invention include those of the general formula I:

R - N = O (I)

wherein R represents an unsubstituted or substituted alkyl group, preferably a tertiary alkyl group (e.g. tertiary butyl or octyl, or a group C(CH₂ OR')₃ or a group R_x C(CH₂OR')_3-x wherein x is 1,2 or 3, R is as defined below and R' represents a hydrogen atom or an unsubstituted or substituted alkyl or acyl group and in which each R' may be the same or different), and R may be an unsubstituted or substituted (e.g. alkyl-, chloro-, hydroxy-, carboxyl-, cyano-, nitro-, or dimethylamino-substituted) phenyl or naphthyl group (e.g. phenyl, tetramethyl-phenyl, pentamethyl-phenyl, trichloro-phenyl, hydroxy-phenyl, di-tert-butyl-hydroxy-phenyl, dimethylamino-phenyl and hydroxy-naphthyl) or a heterocyclic aromatic or reduced heterocyclic (e.g. piperidinyl) group.
Particularly preferred C-nitroso compounds are the tertiary alkyl nitrosos, particularly 2-nitroso-2-methylpropane and 2-nitroso-2,4,4-trimethylpentane. These compounds are highly effective as U.V. stabilisers and act as melt stabilisers. At the same time the compounds are much cheaper to produce than conventional commercially available stabilisers.
In choosing the stabiliser to use according to the present innvention, regard has to be taken of the physical properties of the stabiliser. Thus for example in many applications the use of a stabiliser which is highly coloured needs to be avoided. Also for example stabilisers which are lost to the system e.g. by being volatile under the processing conditions should be avoided.
The stabilisers according to the present invention are in general known compounds and may be obtained by methods known per se. For example they may be obtained as follows:
Nitroso alkanes may be prepared by oxidation of the corresponding amines with hydrogen peroxide by the methods of Stowell (J. Org. Chem., 36, 3055 (1971)).
Aromatic nitroso compounds may be prepared by oxidation of the corresponding amine with hydrogen peroxide by the method of Richard and Bayer (J. Am. Chem. Soc., 82, 3455 (1960)).
The thermoplastic polymers of the stabilised compositions according to the present invention are in particular polyethylene and polypropylene, and polyvinyl chloride. However there may also be used ethylene/propylene copolymers.
One interesting aspect of the present invention is the use of stabilisers according to the invention in conjunction with other stabilisers. As noted above the C-nitroso compounds act in particular as U.V. stabilisers and in addition have melt stabilising activity. By using other stabilisers in conjunction with the stabilisers according to the invention, stabilising effect may be broadened and increased. Particularly interesting is the use of stabilisers according to the invention in conjunction with other stabilisers and synergism then observed. For example there may be included in the compositions according to the invention U.V. stabiliser compounds to give high U.V. stability. Such U.V. stabilisers compounds include Tinuvin 770, Cyasorb UV 531 and Tinuvin 327. [Tinuvin 327, ex Ciba-Geigy AG, is a benztriazole of the formula:

tBu represents tertiary butyl.] Such U.V. stabilisers can be used in conjunction with the C-nitroso-containing stabilisers to give compounds of high U.V. stability. Alternatively, it may be desirable if for example thermal antioxidative stability is required to use a thermal antioxidant together with the C-nitroso-containing stabiliser. It has been found that when stabilisers according to the invention are used in conjunction with for example U.V. stability of the final composition is higher than might be expected on the basis of the summed activities of the individual stabilisers when used alone. Also a synergistic effect may be observed when a thermal antioxidant is used with a C-nitrosamine.
The processing conditions under which the compositions according to the present invention are prepared are important to the obtaining of stabilisation according to the present invention. As explained above, it is believed that radicals formed mechanochemically under the high shear conditions of processing react with the stabilisers according to the present invention to result in the formation of a nitroxyl radical. If mixing is carried out under conditions which do not result in nitroxyl formation the stabilisation according to the invention is not obtained. Indeed observations indicate that the amount of stabilisation obtained depends on the level of nitroxyl radical formation. Generally the composition according to the present invention will be prepared by simply mixing the thermoplastic polymer and stabiliser ingredients using a conventional extruder or injection moulding machine. The importance of the processing operation has been demonstrated in that the effectiveness of the present stabilisers in stabilisation is dependent upon processing time. Thus generally there is an optimum processing time to give optimal stabilisation and if this time is not reached or is exceeded, reduced stabilisation will be obtained.
In the commercial formulation of polymer compositions, processing times are often fixed by the residence time of the machinery being used. In these circumstances it may be advantageous to produce a master-batch composition according to the present invention having a high content of stabiliser. This master-batch may be obtained under processing conditions which ensure that optimum stabilisation is obtained and then may be used as additive to a thermoplastic polymer in a conventional extruder or injection moulding machine. In this way the optimum stabilisation of the present invention may be obtained without the residence time in the conventional extruder or conventional moulding machine being critical. The processing time used in the production of the master-batch on the other hand can be readily adjusted and is chosen such as to provide optimum stabilisation. The master-batch may suitably be advantageous to introduce additional radical generators, for example, dialkyl peroxides, during the processing operation.
The stabilisers according to the present invention are generally used at up to 1% by weight of the thermoplastic polymer composition. The preferred range is usually 0.05 g to 0.5 g stabiliser per 100 g of polymer. In the case of master-batches however the concentration of stabiliser will be considerably greater. For example between 2 and 15% by weight of the thermoplastic polymer composition.
The following Examples illustrate the invention.

EXAMPLE 1

Tests were carried out using the following compounds:

U.V. Stabilisation

The C-nitroso compounds above were compounded under polymer processing conditions in a RAPRA Torque Rheometer with polypropylene at 180°C and at the rate of 10⁻³ moles/100g except where indicated otherwise using the processing times indicated below.
In each case, the composition was formed by compression moulding into a film which was then exposed to a sun lamp black lamp (SB) which simulates sunlight. The time before the film became brittle was measured.
The results obtained were as follows:

Melt Stabilisation

The melt stabilisation of polypropylene obtained with C-nitroso compounds according to the invention was measured and compared with that of polypropylene without melt stabiliser.
Unlike polypropylene without melt stabiliser, in which the melt flow index doubled within 5 minutes on processing in a RAPRA Torque Rheometer at 180°C, the C-nitroso compounds showed an induction period before any change occurred. The times taken (mins) to the end of the induction period at 180°C in a RAPRA Torque Rheometer are as follows:

Nitroso compound Induction period (mins)

tB-NO 20

tO-NO 15

B-NO 25

PMB-NO 20

TCB-NO 10

HB-NO 30

HBB-NO 25

DMA-NO 30

HN-NO 10

Thermal Antioxidant Activity

The C-nitroso compounds were compounded at the rate of 10⁻³ moles/100g polypropylene and processed at 180°C. A film was formed and the time to embrittlement (single cell air oven at 140°C) was measured.
The results obtained were as follows:

Synergism with Phenolic Antioxidants

It can be seen from the above table that the nitroso alkanes have little thermal antioxidant activity. However, it was found that in combination with a commercial hindered phenol, synergism was observed and a very thermally stable formulation was obtained. Thus Irganox 1076 + tB-NO each at 5 x 10⁻⁴ moles/100g give an embrittlement time at 140°C of > 41 hours. [Irganox 1076, ex Ciba-Geigy, is of the formula:

tBu representing tertiary butyl.]

Nitroxyl Concentration

The concentration of nitroxyl radicals (10⁻⁵mole/100g) in polypropylene after processing with the nitroso compounds in a closed mixer (180°C) was measured. The initial concentration of C-nitroso compound was 10⁻³mole/100g. The results were as follows:

EXAMPLE 2

The synergistic effects of stabilisers used according to the present invention with conventional U.V. stabilisers were tested by measurement of the U.V. stabilisation as in Example 1 of polypropylene compositions prepared using such mixtures of stabilisers. By way of control embrittlement times of polypropylene containing the stabilisers alone were also obtained. The results were as follows:

Additive	Concentration (x10⁻⁴ mole/100g)	Embrittlement time (hours)
Control (no additive)	-	85
tB-NO	5	410
Cyasorb UV 531	5	340
Tinuvin 770	5	700
B-NO	5	210
Tinuvin 770 + tB-NO	5+5	> 2000
Tinuvin 770 + B-NO	5+5	2000
Cyasorb UV 531 + tB-NO	5+5	> 2000

Claims

A process for preparing a stabilised polyethylene, polypropylene, ethylene-propylene copolymer or polyvinyl chloride thermoplastic polymer composition which process comprises mixing the polyethylene, polypropylene, ethylene-propylene copolymer or polyvinyl chloride
with a C-nitroso-containing compound as stabiliser under polymer processing conditions such that nitroxyl radicals are formed.
A process according to claim 1 wherein the thermoplastic polymer is a polyethylene, polypropylene, or polyvinyl chloride.
A process according to claim 1 or claim 2 wherein there is used as stabiliser a C-nitroso-containing compound of the general formula I

R - N = 0 (I)

wherein R represents an unsubstituted or substituted alkyl group, an unsubstituted or substituted phenyl or naphthyl group, or a heterocyclic aromatic or reduced heterocyclic group.
A process according to claim 3 wherein there is used as stabiliser a tertiary alkyl nitroso compound.
A process according to claim 3 wherein in the general formula (I) R represents a tertiary butyl or octyl group, a group C(CH₂OR')₃ or a group R_x C(CH₂OR')_3-x (wherein x is 1, 2 or 3, R is as defined below and R' represents a hydrogen atom or an unsubstituted or substituted alkyl or acyl group and in which each R' may be the same of different), and R may represent an unsubstituted or alkyl-, chloro-, hydroxy-, carboxy-, cyano, nitro- or dimethylamino- substituted phenyl or naphthyl group.
A process according to any one of the preceding claims wherein there is used up to 1% by weight of the stabiliser.
A process according to any one of claims 1 to 6 wherein an additional stabiliser is also mixed into the thermoplastic polymer.
A process according to claim 7 wherein the additional stabiliser is a thermal antioxidant and/or U.V. stabiliser.
A process according to any one of claims 1 to 5 wherein there is first prepared a master-batch composition by mixing a thermoplastic polymer with a nitrogen-containing stabiliser under polymer processing conditions such that nitroxyl radicals are formed and mixing the master-batch composition with further thermoplastic polymer.
A composition prepared by the process claimed in any of claims 1 to 9.